There is really no need for gaskets on exhaust manifolds. If you doubt that, consider that the factory didn't use them either.
As for tight, the shop manual can provide a torque value. As for myself, I draw them down just snug + about 1/8 turn more.
Charlie, CMSGT USAF Ret. 65 Hi-Po F/B, Mustang Owners Club of Austin (MOCA) TX, Member MCA #27479. Now working a '69 M code mach 1.